1. Field of the Invention
This invention relates to a silver halide photographic light-sensitive material which forms a direct positive photographic image, and more particularly, to a photographic light-sensitive material in which a novel compound is contained as a fogging agent in a photographic emulsion layer or another hydrophilic colloid layer(s).
2. Description of the Prior Art
In the field of silver halide photography, photographic processes which can produce photographic images without the formaton of a negative image or without the intermediate treatment for obtaining a negative image are called direct positive photographic processes, and photographic light-sensitive materials and photographic emulsions used in such photographic processes are called direct positive light-sensitive materials and direct positive photographic emulsions, respectively.
Various types of direct positive photographic processes are known, but a process in which previously fogged silver halide grains are exposed to light in the presence of a desensitizer and then developed, and a process in which a silver halide emulsion having sensitivity specks mainly in the interior of the silver halide grains is exposed to light and then developed in the presence of a fogging agent are the most useful. This invention is concerned with the latter process. Silver halide emulsions having sensitivity specks mainly in the interior of silver halide grains and forming a latent image mainly in the interior of the grains are called silver halide emulsions of the internal latent image type.
A method of directly forming a positive image by subjecting a silver halide photographic emulsion of the internal latent image type to surface development in the presence of a fogging agent, and photographic emulsions and light-sensitive materials used in such a method are well known and are described in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318 and 3,227,552, British Pat. Nos. 1,011,062 and 1,151,363, and Japanese Patent Publication 29,405/68.
In the above method of directly forming a positive image, the fogging agent may be added to a developer, but better reversal characteristics can be obtained when the fogging agent is added to a photographic emulsion layer or other layers of the light-sensitive material to adsorb the agent on the surface of the silver halide grains.
Fogging agents which are added to a silver halide emulsion or other layers of a light-sensitive material include the hydrazine compounds described in U.S. Pat. Nos. 2,563,785 and 2,588,982. However, these hydrazine compounds, when added to an emulsion layer, must be used at a quite high concentration (e.g., about 2g per 1 mol of silver). Moreover, since the fogging agent is transferred from the emulsion layer to a developer during development, the concentration of the fogging agent in the emulsion changes, resulting in a variation in maximum density (in the unexposed portions). Further, in multilayer color light-sensitive materials, an unequal fogging effect among the emulsion layers is produced.
Known fogging agents which are free from the above defects include those heterocyclic quaternary salt compounds described in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738 and 3,759,901. However, silver halide emulsions, in many cases, contain a sensitizing dye for spectral sensitization. In particular, for color light-sensitive materials, a layer sensitive to blue light and also layers respectively sensitive to green light and red light are indispensable, and the emulsions of the green-sensitive layer and red-sensitive layer necessarily contain sensitizing dyes.
When a fogging agent together with a sensitizing dye for green light or red light are incorporated into a direct positive emulsion, competitive adsorption on the silver halide occurs between the sensitizing dye and the quaternary salt fogging agent. Therefore, when the fogging agent is employed in an amount necessary for forming the desired nuclei, spectral sensitization is inhibited, and on the other hand, when the spectrally sensitizing dye is used in a concentration sufficient to obtain the desired spectral sensitization, the formation of fog nuclei is inhibited.
The use of a sensitizing dye containing, in the dye molecule, a substituent having a fogging (nucleating) effect as described in U.S. Pat. No. 3,718,470 is known as a method of overcoming the above defect.
However, this method of making a single molecule have a fogging effect and a spectral sensitizing effect is disadvantageous, e.g., in that the use of an amount adequate for spectral sensitization results in an unsatisfactory fogging effect, and on the other hand, the use of a sufficient amount to produce a fogging effect is unsuitable for spectral sensitization.
To solve the above problems, fogging agents which are more readily adsorbed on silver halide and perform the desired formation of nuclei by their use in such an amount which does not inhibit spectral sensitization are required.